Internal combustion engines such as gasoline engines, diesel engines, and gaseous fuel-powered engines exhaust a complex mixture of air pollutants. Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of pollutants emitted to the atmosphere from an engine is regulated depending on the type of engine, size of engine, and/or class of engine. In addition, fuel costs have risen significantly in recent years. As a result, efforts are being made to produce hybrid electric vehicles with electric powertrain systems that supplement internal combustion engines with electric power to reduce emissions and operating costs (i.e. fuel costs) associated with internal combustion engines.
A typical electric powertrain system for a hybrid electric vehicle includes an internal combustion engine and one or more electric traction motors operable to drive a traction device of the vehicle so that the vehicle can traverse a ground surface. The internal combustion engine generally produces a mechanical output (e.g. a rotation of a crankshaft), which is received by an AC alternator. The AC alternator is driven by the mechanical output of the engine to produce an alternating current (“AC”) output, with a frequency dependent on the rotational speed of the crankshaft. A first set of power electronics (i.e. a rectifier) receives the AC output and converts it to a direct current (“DC”) output. This DC output is then used to drive the traction motors in one of two ways. In a first example, the traction motors are DC traction motors that can be driven directly by the DC output of the first set of power electronics. In a second example, the traction motors are AC traction motors. In this case, the DC output is passed through a second set of power electronics (i.e. an inverter) that converts the DC output to an AC output with a desired frequency. This AC output is then used to drive the AC traction motors at a speed related to the frequency of the AC output. When the hybrid electric vehicle is operated in a dynamic braking mode (e.g. braking, decelerating, downshifting, etc.), the traction motors operate as generators driven by the traction devices themselves to produce current directed back towards the engine. This reverse current is shunted away from the powertrain to a grid of resistors that dissipate the current as heat energy.
Efforts are being made to make use of this dynamic braking mode current to supplement the electric powertrain system, rather than being dissipated and wasted. One possible use of the dynamic braking mode current was disclosed in U.S. Pat. No. 5,351,775 (“the '775 patent”) by Johnston et al. on Oct. 4, 1994. Specifically, the '775 patent disclosed an electric powertrain system having bidirectional thyristor-type converters controlled to allow the dynamic braking mode current to flow back to an engine. More specifically, the engine is connected to drive an AC generator to produce an AC output current that is passed through an AC power grid to a field converter and two armature converters. The converters translate the AC output to a DC output used to drive two DC traction motors. Additionally, the electric powertrain of the '775 patent provides a connection to a DC trolley line so that DC power from the trolley line can be used to supplement the powertrain system. More specifically, the DC power from the trolley line is converted to AC power by a bidirectional trolley/retard converter, which is then fed directly to the AC power grid. The connection to the DC trolley line also includes a set of retarding resistors.
When the electric powertrain system of the '775 patent is operated in a retarding mode, the DC traction motors act as generators sending DC power through the bidirectional field converter and armature converters. In this direction, the bidirectional converters are operable to convert the DC power to AC power. This converted AC power is delivered through the AC power grid to drive the AC generator to act as a motor driving the engine. In this manner, the fuel consumption of the engine is reduced. Additional AC power is delivered to the bidirectional trolley/retard converter for conversion back to DC power and dissipation as heat by the retarding resistors.
While the electric powertrain system of the '775 patent may make use of the energy produced by traction motors in a dynamic braking mode, it may be expensive. That is, because the engine is coupled with an AC alternator, the field converter, armature converters, and trolley/retard converter must each be capable of converting DC power to AC power in one direction and AC power to DC power in a second direction. The amount and types of electronic components necessary to manufacture these bidirectional converter circuits may make them prohibitively expensive.
Further, because the electric powertrain system of the '775 patent uses an AC generator to convert the mechanical power output of the engine to electrical power, it may inefficiently utilize under-hood space. More specifically, AC generators are generally larger than their DC counterparts. That is, an AC generator rated to convert a certain amount of horsepower to electrical power may be larger than a DC generator rated to convert the same amount of horsepower to electrical power. Further, AC generators rated to convert lower frequencies of mechanical rotation to electrical power are generally larger than AC generators rated to convert higher frequencies of mechanical rotation. That is, in order for the AC generator of the '775 patent to make efficient use of all frequencies of mechanical rotation outputted by the engine, the AC generator's size must be larger than an AC generator rated only for higher frequencies. Thus, the AC generator of the '775 patent may take up a relatively large amount of space in an associated vehicle.
The disclosed electric powertrain system is directed to overcoming one or more of the problems set forth above.